Insulating coating compositions for applying to transformer wire are disclosed in commonly assigned U.S. patent applications Ser. Nos. 697,838 now abandoned and 889,889. The electrically insulating coating materials disclosed in the aforementioned applications comprise the reaction product of a mixture of polyvinyl acetal, phenol aldehyde resin and epoxy resin in a particular range of proportions. The mixture is formed into a powder for applying to wire without the use of solvents. The coated wire is then heated to cure and harden the applied coating.
The aforementioned resin composition requires the addition of a prescribed amount of a curing agent which is designed to react and harden the coating into a tough flexible adherent layer during a thermally controlled coating process. The premixed powder including the curing agent is electrostatically applied to wire which is then heated to cause the resin composition to react with the curing agent.
A preferred method of manufacturing the powder described in the above patent applications is to preblend the ingredients at room temperature and then to pass this premix through an extruder in order to prepare a homogeneous blend. Typically, the extruder will be operated at a temperature of 125.degree. to 150.degree. C. The extrudate is cooled and chopped into small granules which are subsequently ground to a fine powder of suitable particle size distribution.
The ingredients of the premix will normally contain moisture which has been adsorbed from the atmosphere. It has been discovered that when such a premix containing moisture is passed through the heated extruder excessive loss of curing activity takes place, presumably by reaction of the curing agent with water, rendering it nonactive. The resultant powder when applied to wire does not produce a coating having the desired properties, specifically it is lacking in flexibility. If on the other hand the premix is dried prior to extrusion a much smaller loss of reactivity takes place and the resultant powder produces a coating having the desired properties including flexibility.
The amount of water present in the undried premix and the harmful effect will depend on the atmospheric relative humidity. It has been discovered that the moisture content may commonly be as high as 2.5% and it must be reduced to less than 0.5% to produce satisfactory coatings.